Isolation transformers are used in application where low level logic components activate high power switching devices. While the power required to drive the logic level components is relatively low, the voltages and load currents found across power switching devices are often much higher than the low level circuit can withstand. In situations where high voltages and currents propagate to the low level logic components, damage to the logic level components could result. To prevent such a situation, and to provide noise immunity, isolation transformers are frequency employed to provide the necessary electrical isolation between the logic components and the power switching device. In addition, because isolation transformers may be used in drive applications where a controller activates power switching devices, it is desirable to be able to alert the controller to overvoltage or overcurrent conditions present at any of the switching devices.
Accordingly, an isolation and fault detection apparatus must provide both sufficient isolation between the high voltage and low level logic components and a method of fault detection to alert the system controller. Furthermore, any isolation and fault detection apparatus must operate at a speed sufficient to allow the controller to activate the power switching device at a desired rate. Present methods of isolation and fault detection require two isolation transformers. One isolation transformer is an input device activated by the logic level components to energize the power switching device. A second isolation transformer is driven by circuits that sense overvoltage or overcurrent conditions at the power device and indicates such a fault condition to the controller. The logic level side of the second isolation transformer is monitored to determine if an error condition has occurred on that power device. It is elimination of the second isolation transformer to which this invention is directed.